Altraloth (Mezzoloth)

An Altraloth is a Yugoloth that has undergone a physical alteration at the hands of a coven of Night Hags. The resulting fiend is dependent partially on what it once was before its transformation, but is always significantly more powerful than any normal Yugoloth. 

The Lowliest of the Supreme. Mezzoloths reside at the very bottom of the Yugoloth hierarchy, and as such remain largely as the lessermost of the Altraloths. Because of this, a Mezzoloth will very rarely ever be put through this ritual unless the coven performing it has specific need for one. Or, perhaps, if the Mezzoloth manages to gain the coven's favor in some way.

The Cost of Power. An Altraloth is bound by the coven of Night Hags who created it until it has repaid its agreed-upon debt to them. The Altraloth cannot betray the coven until it has completed its duties, and it will perish should any of the coven die before the end of its term. As such, and Altraloth will do everything within its power to protect its masters until the day of its release. After that, the newly-freed Altraloth will go on to pursue its own ends.

Varying Anatomy. No two Altraloths, even those from the same base species, are exactly alike. Choose any number of traits from the list below, adding them to the Altraloth's stat block. For every trait added, give the Altraloth an extra hit die to its total HP. For every 2 traits added, raise the creature's Challenge Rating by 1.

1. Grafted Wings:  Altraloth gains a flying speed equal to its walking speed. If it already possesses a flying speed, it is increased by the same amount.

2. Infernal Blood: The Altraloth's blood carries the blazing heat of Avernus. It is immune to Fire damage, and its melee attacks deal an additional 7 (2d6) fire damage

3. Abyssal Essence: The Altraloth's rebirthing brew was enhanced with the essence of a demon lord. It adds the following spells to its Innate Spellcasting feature:

At Will: Insect Plague, Primal Savagery(17th level), 

3/day each: Summon Lesser Demons, Confusion, Enervation

1/day each: Maddening Darkness, Eyebite

4. Thick Hide: The Altraloth's thickened external hide or shell raises its AC by 2.

5. Carceri's Bindings: The Altraloth's body is wrapped in chains forged in Tartarus, which leap forth from its body to bind foes in an inescapable demiplanar donjon within itself. It adds the Imprisonment spell to its 1/day innate spells. It can only imprison one creature at a time, freeing a captured creature in the place of the new one should it imprison a new target. A creature trapped by this spell is freed if the Altraloth is slain permanently.

6. Fragment of Limbo: The Altraloth has a fragment of Limbo's Spawning Stone embedded in its head. With this, it can control any Slaad that has a Control Gem as though it were in possession of the gem, provided it can see the Slaad in question. Additionally, it can summon a Death Slaad with a control gem once per long rest.

7. Acheron's Armaments: The Altraloth bears markings tying it to one of the legions of Acheron. It scores a critical hit on a roll of a 19 or 20. Additionally, if it reduces a creature to 0 hit points, it gains 10 temporary HP.

8. Breath of Pandemonium: The Altraloth's lungs are perpetually filled with flowing air captured from the plane of Pandemonium. It can hold its breath indefinitely. As a Legendary Action (Costing 2 Actions), the Altraloth can exhale chaos-infused wind in a 60-foot cone. Creatures other than fiends in this cone must succeed a DC 17 Constitution saving throw or take 21 (6d6) bludgeoning damage and be thrown up to 60 feet from the Altraloth, suffering fall damage for the distance thrown. A creature who fails the saving throw by 5 or more is also deafened for 1 minute. A creature who passes the save takes half damage and isn't launched.

9. Unending Gloom of Hades: The Altraloth's being is suffused with the crushing gloom of Hades. While in dim light or darkness, the Altraloth is resistant to magical Bludgeoning, Piercing and Slashing damage. It also no longer needs to concentrate on its innate Darkness spell.

10. Flesh of the Witch's Flock: The making of the Altraloth included the sacrifice of a portion of the hags' troll servants, whose flesh was fused with the Yugoloth's own. The Altraloth regains 15 hit points at the start of its turn. If the Altraloth takes Radiant damage, this trait doesn't function until the start of its next turn. The Altraloth dies only if it starts its turn with 0 hit points and doesn't regenerate.